The immune system diversity of the critically endangered Wyoming toad is examined in a new study from North Carolina State University, which indicates that genetic bottlenecks can affect a species’ ability to respond to novel infections. The findings could help endangered animal species develop captive breeding tactics.
Due to reasons like as habitat degradation and fungal illness, the Wyoming toad, Anaxyrus baxteri, had a catastrophic population fall in the latter half of the twentieth century.
In the 1990s, the toad was introduced into a captive breeding program in trying to rescue the species. According to scientists, the toad has a wild population of only 400 to 1,500 creatures, making it severely endangered.
“Population reduction in this species created a genetic bottleneck to begin with, meaning the level of genetic diversity is already very small,” says Jeff Yoder, professor of comparative immunology at NC State and co-corresponding author of a paper describing the work.
“This is the first study to look specifically at genetic diversity in the immune systems of these toads and how it could impact them as a population.”
RNA sequencing was performed on immunological tissues from three healthy, retired Wyoming toad breeders by Yoder and co-corresponding author Alex Dornburg of the University of North Carolina at Charlotte.
MHC genes are some of the most rapidly evolving sequences in the genome. So in a healthy population, there’s a lot of variety that gets passed along to descendants, enabling the species at large to adapt to different pathogens. However, if disease survivors do so because of their MHC, then that group would have a similar MHC.
Kara Carlson
The samples were gathered by study co-author Michael Stoskopf, who was a member of the Wyoming Toad Recovery Implementation Team, which was formed in 2008.
“We were focused specifically on sequences encoding toll-like receptors TLRs and the proteins of the major histocompatibility complex, or MHC, expressed in these tissues,” says Kara Carlson, first author of the study and current Ph.D. candidate at NC State. “These sets of genes are major components of the immune system.”
TLRs are immune system first responders that are comparable or well-conserved across species. On the other hand, the MHC is a big and diverse set of genes that differs between animals and individuals. It can help researchers figure out why one population is more resistant to a virus than another.
“MHC genes are some of the most rapidly evolving sequences in the genome,” Carlson says. “So in a healthy population, there’s a lot of variety that gets passed along to descendants, enabling the species at large to adapt to different pathogens. However, if disease survivors do so because of their MHC, then that group would have a similar MHC.”
Although all of the Wyoming toads brought into captivity to save the species were able to fight the fungus that had ravaged the population, this could suggest that their immunological diversity has been decreased.
The TLR and MHC of the three Wyoming toads were compared to those of a common toad and a cane toad, as well as samples from a common toad and a cane toad. Despite going through a similar genetic bottleneck, both the common toad and the cane toad have higher MHC variety than the Wyoming toad.
“The small sample size in this study which was unavoidable due to the endangered status of the toad nevertheless lays an important framework for conservation,” Carlson says.
“Amphibians, in general, don’t have as many genomic resources as other organisms,” Yoder says. “And captive breeding from a small population further decreases genetic diversity. But while these toads may be better protected against the fungal infection that nearly wiped them out, they may not be equipped to deal with new pathogens down the road.”
“While we weren’t necessarily surprised by the lack of immunogenic diversity in the Wyoming toad, it does spark an important question,” Dornburg says. “How equipped are other species of conservation concern for a battle with an emergent pathogen?”
“By understanding the genetic diversity of the immune system we can inform captive breeding to increase the chance of a species to resist disease in the wild,” Yoder adds. “Studies like this one are invaluable for captive breeding practices going forward.”
The National Science Foundation (NSF), funded the study of the North Carolina State University Research and Innovation Seed Funding (RISF) Program, and the North Carolina State University Center for Comparative Medicine and Translational Research (CCMTR).
